Universal Serial Bus (USB) is known as a standard of communication between an information terminal such as a computer and an imaging apparatus. Moreover, recently, Picture Transfer Protocol (PTP) has been proposed by Photographic and Imaging Manufactures Association, INC. (PIMA) as a communication standard in an upper layer of USB which has been created based on a still image rank interface of USB.
The standard regulates a data transfer system for a digital camera, a scanner and the like. By the standard, a user can easily load a data file of a pick-up or scanned image into a personal computer without being conscious of a directory structure in which an image file is stored. When a computer and a digital camera are connected to each other and they mutually exchange data based on a communication protocol PTP, in order that a computer may obtain the information of one stored object (data such as an image, a sound, and a text), the computer first transmits GetObjectInfo. After that, the computer executes one sequence of receiving Object data and receiving Ack.
Hereupon, the information obtainable by GetObjectInfo is not the object itself, but the attribute information of the object such as the file name thereof, the file size thereof, the directory structure thereof, and the type of a file thereof can be obtained. That is, in order to obtain the data of a certain object, obtaining the attribute information obtained by GetObjectInfo and obtaining the data body thereafter are performed.
On the other hand, when a plurality of objects of a storage medium is exchanged, the OS of the computer requests the attribute information relative to the data stored in the storage medium and the information of the folder structure at first in order to know the folder structure of the storage medium held by the digital camera.
Consequently, in order to obtain the attribute information relative to all the objects of the data such as images and sounds stored in the digital camera, exchanges of the above-mentioned command are to be executed over again for the number of the stored objects (see Japanese Patent Laid-Open Publication No. 2004-15234).
As described above, in order that the OS of the computer may obtain the information of all the images stored in the digital camera using PTP, it is necessary to perform the communication mentioned above for at least the number of the stored images.
That is, it is found that the time necessary for obtaining the information of all the images that the digital camera stores is simply proportional to the number of the stored images.
In particular, on the image reception side (for example, the PC), if the transmission and the reception of data on PTP cannot be performed until the image receiving side has obtained the information such as the attribute information and has reproduced the folder structure thereof, the time taken until the image receiving side reproduces the folder structure and the directory structure becomes long.
That is, when a large-capacity memory card is installed in the digital camera and the number of the images and the other objects stored in the memory card is very large, the time until a response to a user is initiated after the computer and the digital camera have been logically connected becomes extremely long. For this reason, usability becomes bad and this has been also the weak point of the communication of PTP.